On Chanukah, the main mitzvah, or commandment is to light the Hanukkah menorah at sundown. This is to remember the miracle that the oil that was lit on the menorah was only enough to last a single day but ended up lasting for eight complete days.
One of the traditions regarding lighting candles is to light oil like what was done in the Temple. However, we are not allowed to use a menorah that has seven branches like the one in the Temple –— which the Maccabees lit after expelling the Greeks from Israel.
Instead, we light a menorah with eight branches, which is also known as a Chanukiyah. The simplest explanation for why we light an eight-branch menorah is because each candle represents one of the eight nights over which the miracle lasted. However, there is a more complicated reason as to why the mitzvah is performed as it is.
To understand why we may light with oil and may not use a seven-branch menorah, we must look at Jewish Law, especially those laws involved with the building of the Mishkan and Beit HaMikdash, the Tabernacle and the Temple, respectively.
The Bible at the end of the Book of Exodus states that those who construct an imitation of any of the items in the Mishkan and Beit HaMikdash will suffer a punishment of spiritual excommunication. However, this punishment is mentioned by a few items, so in theory it would only apply to those items. However, the Talmud forbids it and there is much discussion as to what this means.
There are three views as to what this means. The first view is that only an exact replica is forbidden and therefore any deviation is permitted. The second view is stricter and states that any imitation that would be a looser imitation but is still very similar to that used in the either the Mishkan or Beit HaMikdash is forbidden. The last opinion is the most stringent and forbids any type of imitation.
So how does this apply to a menorah? According to these opinions, we have a real quandary as to what menorah we can use. According to the first view, in theory a seven-branch menorah would be fine as long as it does not have the same types of decorations present on the menorah from the Mishkan or Beit HaMikdash, such as flowers, knobs and cups.
The second view would pretty much forbid any seven-branch metal menorah, even if it was decorated differently and was much smaller. The third view would forbid all seven-branch menorahs made of metal, regardless of it shape. However, it is important to note that since the vessels used in the Beit HaMikdash and Mishkan were made of metal, it is specifically a menorah made of metal that is prohibited. However, a menorah made of porcelain or wood would be permitted.
So how do such a wide range of opinions apply practically?
The major authorities in Jewish Law as well as the important commentaries on the Torah rule according either the second or third opinion. Thus, for those people who want a menorah that specifically similar to that used in the Beit HaMikdash, there is a serious problem because they cannot have such a menorah. Therefore, the custom arose to add a branch to the menorah, thus making a menorah with a central branch and eight branches coming out from the central branch, four on each side. As an alternative, someone who wants a menorah that is very similar to that in the Beit HaMikdash could purchase a porcelain or wood menorah and thereby circumvent the prohibition.